Jimmie G. is an animated, intelligent man as long as he talks about his life before 1945. He can remember nothing since then, not even an event of five minutes ago. Dr. P. was a musician who, late in life, lost his ability to recognize familiar faces or objects: He could identify a rose only as "a convoluted red form with a linear green attachment," and a glove as "a continuous surface infolded on itself" with "five outpouchings." At 88, Mrs. O'C. began hearing Irish songs of her youth, an unsolicited concert that played repeatedly in her mind. Stephen D., a medical student who habitually used amphetamines and cocaine, dreamed he was a dog and awoke with an enhanced olfactory sense. For several weeks he was able to recognize friends and patients, to detect fear and contentment, solely by smell.

These people—whose stories neurologist Oliver Sacks recounts in The Man Who Mistook His Wife for a Hat and Other Clinical Tales (Summit Books, $15.95)—dwell in the twilight margins of human experience. Tumors or lesions of the brain have disrupted their lives, often profoundly. Sometimes what is deranged is so fundamental that only through its absence is it noticed: for instance, proprioception, the sense of the position or movement of the parts of one's own body. A young man whom Sacks saw many years ago, for example, was certain that one of his legs—which seemed totally alien—was a dissected limb slipped into his bed as a macabre joke. Many neurological patients, derailed from society's main track, have been shunted into state hospitals or chronic-care nursing homes, the institutions in which Sacks, 52, has been practicing for two decades. "In places like these, where most ambitious young doctors wouldn't be seen dead, there are great challenges and explorations to be undertaken," he says. "One isn't pushed for time in these asylums. I don't have to see 30 people an hour or immediately come up with a diagnosis." As for the rewards, he says, "I love to discover potential in people who aren't thought to have any."

Sacks is best known for a previous book, Awakenings, about a group of patients who had long appeared hopeless. At the outset of his career in clinical medicine, in 1966, Sacks was assigned to Beth Abraham, a large hospital in the Bronx. There he found about 80 survivors of a crippling epidemic of encephalitis lethargica, known as "sleeping sickness," a viral disease that was prevalent from the time of World War I into the 1920s. Many of these patients at Beth Abraham—one of the few large groups of survivors in the world—had sat in the wards for 30 or 40 years, frozen like statues in a grotesque trance. Fortuitously, a few months after Sacks arrived, the publication of a now famous scientific paper announced the extraordinary effect of the drug L-dopa on patients suffering from Parkinson's disease. Since the immobility and uncontrollable movements of the postencephalitics at Beth Abraham seemed akin to parkinsonism, L-dopa held the promise of awakening these patients from their neurological underworld. In March 1969 Sacks began to administer the drug to his patients.

Their response, he later wrote, "had an explosive quality, as of corks suddenly released from a great depth." Patients who had been enthralled for decades suddenly began to move, to speak, to act; individual personalities emerged. But the drug did not provide a simple cure. Almost all the patients, once quickened, grew increasingly overexcited. The response could not be reliably regulated through dosage, so in some cases the medication had to be discontinued. Sacks felt that the patients' reactions depended not only on the severity of the brain damage, but on the quality of life that greeted them upon awakening. Those patients who could practice a trade or visit with relatives responded better to the medication. For one man, who had been a cobbler, Sacks and the staff set up a workshop; his condition improved remarkably.

By relating 20 case histories in Awakenings, Sacks examined how different individuals coped with both their illness and its treatment. First published in 1973, the book attracted many eminent readers outside the medical community. W.H. Auden declared Awakenings" masterpiece," and Harold Pinter wrote a one-act play (A Kind of Alaska) inspired by one of the case histories. Other neurologists were less enthusiastic, viewing the book as anecdotal, not scientific. "The kind of science in vogue today is more biochemically and technologically oriented," explains Dr. Isabelle Rapin, a professor of pediatric neurology at the Albert Einstein College of Medicine, where Sacks currently works. "The entire trend of medicine is to process as many patients as efficiently as possible. Dr. Sacks is a humanist as much as a neurologist, and that aspect of medicine is not rewarded very much at the present time."

Sacks admits that at times he may exaggerate to intensify his portraits but insists that he only embellishes what is tangential—for instance, he depicts the conventional, if drab, Beth Abraham as a subterranean fortress. The patients' symptoms, he says, he records faithfully. "I lay it on a bit, but I bring things into focus that might not have been seen before. I don't think I essentially distort. I tend to dramatize." He argues that the journal articles of most neurologists obscure the phenomena by reporting only quantifiable test results.

Sacks became painfully aware of the blindness of institutional medicine when he found himself the patient after a fall in Norway in 1974. The muscles and nerves of one leg were severely torn. Along with the injury, he endured a harrowing loss of proprioception: His leg, totally without sensation, seemed no longer to exist. Although this perceptual loss disturbed him as much as the ripped quadriceps, his surgeon, as if wearing blinders, acknowledged nothing but the damaged muscle. Once the physical injury healed and sensation returned, the surgeon signed off—even though Sacks had forgotten how to use the leg. To regain its natural use, Sacks resorted first to music. Listening to a Mendelssohn melody, he found the rhythm of his old walk. Then he visited another, wiser surgeon, who, on learning that swimming was Sacks's favorite exercise, immediately sent him to a public pool. The lifeguard there, in collusion with the surgeon, pushed Sacks into the water and dived in after him, challenging him to a race. Before he had a chance to think, Sacks was swimming, and when he emerged from the pool he was walking naturally.

The whole experience so unnerved Sacks that he waited eight years before writing a book about it. Then, two weeks after completing A Leg to Stand On, he slipped on the ice near his house and suffered a similar tear in the quadriceps of his other leg. This time, though, there was no nerve damage. He underwent surgery within two hours and was helped to stand up soon afterward. No body-image disturbance troubled his convalescence.

Since his parents were both physicians and his three brothers all studied medicine, it would have been remarkable had Oliver, the fourth and youngest child, not entered the profession. Born into a cultured Jewish family—one of his cousins is Abba Eban, Israel's former UN ambassador—he grew up happily near Hampstead Heath in London until World War II. Six-year-old Oliver was evacuated to a boarding school in the country while his parents continued to work in the city. He remembers the school as a Dickensian horror where the children were fed only giant turnips and mangel-wurzels, vegetables usually consumed by cattle. "I don't know if it was a fantasy," he says, "but we all felt that if we didn't bolt the food it would be taken away from us." He saw his parents "too rarely and, above all, unpredictably." After three and a half years he returned to London, and he dates his interest in science, especially inorganic chemistry, to that homecoming. "Studying the periodic table, I felt that this was an eternal system," he says. "The halogens would always be a family, the alkali metals would always be a family, and nothing could ever break them up. After my profoundly disturbing experience in this school, science had the great appeal of order and stability."

During his early medical studies, Sacks was drawn to neurology. "The brain is the most intricate mechanism in the universe," he says. "I couldn't imagine spending my life with kidneys." In 1960, after a London internship, he traveled to Canada and hitchhiked for four months. "Then I dropped into California and fell in love with it," he says. "Seeing Muir Woods and the redwoods made me think it really was a new world." He found a position in San Francisco doing research on Parkinson's disease, and in 1965 went east to the Albert Einstein College of Medicine in New York, where today he is a professor of clinical neurology. "At first I saw myself, foolishly, as a research scientist," he says. "I was always breaking slides and losing preparations. I was terribly clumsy." Thinking he might find clinical work more congenial, a superior told him of three openings: at Beth Abraham, a Bronx state hospital and a New York migraine clinic. Sacks took all three. "And life changed," he says. "Things were real. I suddenly had a delight in living that I hadn't felt for a long time."

The migraine clinic particularly appealed to Sacks because he has suffered from migraines since he was a child; indeed the migraine aura, consisting of sensory hallucinations that often precede the headaches, had contributed to his original interest in neurology. "It gave me an intimate knowledge of how the perceptual world can be disintegrated as the result of a physiological event," he says. Fascinated by the constellation of symptoms that make migraine, like Parkinson's disease, "an organized chaos," Sacks in 1970 published his first book, Migraine: Understanding a Common Disorder, which was reissued in an expanded edition last year.

Sacks wrote Migraine in nine days in September 1968, after he was fired by the clinic director, who objected to a subordinate's presumption in planning such a book. Only after completing Migraine could Sacks concentrate on his next project: administering L-dopa to the postencephalitics at Beth Abraham. He had delayed doing so partly because he needed to raise funds: L-dopa was expensive. He hesitated also, he says, because he feared L-dopa might produce unforeseen deleterious effects. Finally, he confesses, "I may have wanted to keep my patients sick so that I could study them. There may have been a part of me that didn't want my patients ripped away from me and made well overnight. It was a reprobate thought that I tried to push back whenever it came up."

After six frenetic months of treating his patients with L-dopa and monitoring their roller-coaster responses, Sacks went to England for a month. "I returned to the most appalling sight," he says. "There was a bedlam that greeted me. Half the patients were catatonic, the other half had tics and were frenzied. My first thought was that something had gone wrong with the pharmaceutical doses. My second was that there was some kind of flu. In fact, there had been political changes at the hospital. The chief of medicine had been fired. The community of patients had been broken up. No visitors were allowed. The patients were expressing their impotent rage through bodily symptoms."

For institutionalized patients especially, Sacks believes that the solicitude of other people and the chance to engage in meaningful activity—rarely available in large chronic-care hospitals—can contribute at least as much to recovery as any medical treatment. Beth Abraham has been renovated since his arrival, and while he acknowledges that the old 35-bed wards were "a horror," he misses "a kind of farty congeniality, an intimacy that has largely disappeared."

With his hearty embrace of all humanity and his grizzled, unkempt beard, Sacks, despite his English accent, evokes that most American poet, Walt Whitman. Never married, he lives alone in a small, red-shingled house on City Island, a waterfront enclave of the Bronx that is a vestige of an older New York. His shelves are cluttered with books of philosophy and medical files. His edition of the 12-volume Oxford English Dictionary is battered from encounters with his small photocopier, on which he enjoys duplicating definitions of favorite words, a philological predilection evident in his writing style. His other love is for rain forests. "I love Jurassic landscapes, like the tree ferns of Australia, which are carboniferous and prehistoric," he says. "I have a passion for the primitive." In a previous home he filled his front room with dirt and grew tropical plants, running humidifiers for moisture. "Eventually the wallpaper rotted," he says. "You cannot have jungle in your living room."

Although he once accepted private patients, Sacks devotes all his professional time now to hospitals and nursing homes, where the patients are usually elderly and often severely neurologically damaged. He hopes eventually to resume treating younger people, but he doesn't find his current work depressing. "What I can't bear is a situation of cruelty," he says. Since his childhood unhappiness during the war, he has been attuned to "the themes of confinement and release, bondage and freedom, sleeping and awakening," he says. "With my patients, I suppose I am concerned with the freedom or potential of the human spirit against a physiological fate." He wrote his first essay, when he was 7 years old, on Dr. Manette's liberation from the Bastille in A Tale of Two Cities. He called it "Recall to Life"—a fitting overture to the work to which he would one day devote himself.